Add a randomized multi-threaded GC stress test #85

The GC tests cover single features (big stacks, threads, weak hashes, finalizers) but nothing churns them together. A stress program should build and rewire an object graph on several threads, with cross-thread references, weak refs and weak maps, finalizers and zombies, large objects, arrays, strings, closures, GC-free zones and short-lived threads. It should check every object it can still reach. CI should run it in each GC mode: default, generational, moving, always-move, verify, big blocks and check-pointer.

Related: HaxeFoundation/hxcpp#863, HaxeFoundation/hxcpp#872, HaxeFoundation/hxcpp#740 (generational crashes without a reproduction).